1. Pandemic

There's nothing that a pandemic disease loves more than city life, with lots of life forms squashed into close quarters with each other, hopefully without much sanitation. And the time is ripe for a deadly global pandemic, because half of Homo sapiens lives in cities — plus, we have airline travel, so we can ship the pandemic disease around the globe in less than 24 hours. If the disease is virulent enough, humanity could be wiped out in a matter of months.

What most people forget is that human pandemics may not be the most awful way to go. Pandemic crop diseases can be just as virulent as animal diseases, and they can decimate an entire season's worth of staple foods. Because many farmers buy identical strains of staple crops from Monsanto and other corporations, all their crops would be vulnerable to the same infectious diseases. A pandemic could rip through large parts the world's food supply in just one season. If enough crop pandemics struck, humanity could slowly starve to death, or rip itself apart with food riots.

2. Bolide Impact

A bolide is a really large chunk of matter from space, often on fire, that can form a large crater when it smashes into, say, a planet. Often, geologists will describe the event that wiped out the dinosaurs as a "bolide impact." Basically, it's a way of saying "we aren't sure whether that giant thing that smacked into the Earth was a comet or an asteroid, but we know it made a giant crater." There are millions of potential bolides out there, many of them zooming past Earth, and we may not see them coming
.

Early this morning, a fireball blazed over the Chelyabinsk region in Russia, shattering windows,…
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All we need is for a generously-sized bolide to smack into the planet, and — well, we aren't entirely sure what might happen. One possibility is that the planet will be wrapped in fire, burning everyone who isn't deep underwater or underground. Another is that debris from the impact will cloak the planet in dark clouds, cutting off the sun and killing all plant life. Probably a little of both. Bye, bye Homo sapiens.

3. Large Igneous Provinces

You've heard about the caldera volcano under Yellowstone
, which could erupt at some point and release enough lava to pave over Yellowstone Park. But that's nothing compared to the damage that can be done by a large igneous province, which is a volcano that doesn't explode. Instead, a huge crack opens in the Earth's crust — often between tectonic plates — and lava just starts bubbling out, oozing across the landscape, releasing tons of toxic gasses and ash. The thing about an igneous province is that it can keep up the oozing for centuries. And over time, that fills the atmosphere with poisons and wrecks the environment. So humanity wouldn't die out right away, but eventually the planet might kill us with poisons from beneath our planet's crust.

Lurking beneath Yellowstone National Park is a massive underground reservoir of magma, capped by…
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4. Climate Change

Our climate is changing, just as it has throughout the planet's 4.5 billion-year lifespan. Right now, it's changing pretty fast, and that means storm patterns are getting more intense, rainfall is coming erratically, and some areas are suffering major droughts. Though you'd think the scariest part of climate change would be superstorms, the real damage is long-term. Over the next century or more, these changes will wreck our crops faster than a pandemic and leave many people to suffer a famine worse than anything the world has ever seen.

Illustration via NASA

5. Radiation Disaster

The most obvious way that a radiation disaster might wipe out humanity is, of course, from an atomic war. And that's still not out of the question — the people who aren't killed by fire and radiation sickness might die from famine during the nuclear winter that will follow. But cheer up! We might not be the authors of our own demise. We could die from a nearby gamma ray burst, or a blast of highly energetic, radioactive particles streaming out of an exploding star. It would just fry off Earth's atmosphere, and us along with it. You never know when one of these will strike next, so enjoy living the rest of your life knowing that a gamma ray burst could destroy the world at any time.

6. Invasive Species

Humans love to move life forms around the planet — we brought kudzu and plague to the Americas, and brought tomatoes and corn to Europe. We also moved rats and ants all over the world in ocean vessels. And that's just the beginning. The problem is that sometimes these transplanted organisms become invasive species. With no natural predators, they take over every last habitat, eating everything in their paths — or covering everything in a thick blanket of vines (I'm still looking at you, kudzu). Eventually this can cause catastrophic collapse of food webs, or the delicate balance of predator and prey. Plants and animals we depend on for food may no longer be able to grow in environments with invasive species. Cue the food riots and famine and long, slow death of Homo sapiens. Given that we're an invasive species ourselves, this might be a fitting way to go.

When it comes to the apocalypse, you always have to account for a possible black swan event. Perhaps there will be an alien invasion, or we'll discover that the motion of plate tectonics doesn't happen quite as slowly as we thought it did.

One of the most popular black swan theories about human extinction is that we will invent death machines that kill us all. Probably they will be artificial intelligences like Skynet from the Terminator movies, who have been programmed for war and take their job a lot more seriously than we had hoped they would. Or they might just be so intelligent that they decide humans are the equivalent of ants. They don't mean us any harm, but they don't mind stepping on us or exterminating us if we get in the way of whatever incomprehensible things they are doing.